* {
    margin: 0;
    padding: 0;
}

html {
    height: 100%;
}

body {
    height: 100%;
    background: url(./../images/f1_1.jpg) no-repeat center;
    /* 缩放背景图 */
    background-size: cover;
}

.cloud img {
    position: absolute;
    top: 0;
    left: 50%;
}

.cloud img:nth-child(1) {
    top: 20px;
    margin-left: -300px;
    animation: cloud 1s infinite alternate;
}

.cloud img:nth-child(2) {
    top: 100px;
    margin-left: 400px;
    animation: cloud 1s infinite alternate;
    animation-delay: 0.2s;
}

.cloud img:nth-child(3) {
    top: 200px;
    margin-left: -550px;
    animation: cloud 1s infinite alternate;
    animation-delay: 0.4s;
}

@keyframes cloud {
    to {
        transform: translateX(20px);
    }
}